1. Field of the Invention
The present invention generally relates to plasma processing of substrates (e.g., semiconductor wafers or LCD panels) and more particularly, to a method of and an apparatus for pulsed gas injection within a plasma process chamber.
2. Discussion of the Background
A number of steps in the manufacture of semiconductor wafers may use plasma processing. For instance, resist-stripping, etching, depositing, and passivating, may use plasma processing to produce integrated circuits (hereinafter “ICs”) on a substrate. Because the features of the ICs are so small, a uniform plasma density is required for satisfactory resolution thereof. A high plasma density is also necessary in order to maintain process throughputs within a commercially viable range.
Typically, plasmas are established in low-pressure gas environments by causing electrons to collide with individual atoms or molecules, thereby producing additional free electrons and ions. In order to generate the plasma, radio frequency (hereinafter “RF”) power is inductively or capacitively applied to a gas by an inductive or capacitive plasma coupling element, respectively. Examples of inductive coupling elements include conductive, helical, and solenoidal coils that are placed outside of, but in close proximity to, the walls of the process chamber and surround a cylindrically-shaped process chamber. Known inductive plasma generating systems are described in U.S. Pat. No. 5,234,529 (hereinafter “the '529 patent”), issued to Wayne L. Johnson; U.S. Pat. No. 5,534,231 issued to Savas; and U.S. Pat. No. 5,811,022 issued to Savas et al. However, the inductor may also be a planar coil of wire or tubing so as to be placed against the flat top of the cylindrically-shaped process chamber as disclosed in U.S. Pat. No. 5,280,154 issued to Cuomo et al. The coils may be excited by an RF source such that a time varying magnetic field, in accordance with Faraday's Law, becomes associated therewith. The time varying magnetic field produces a time varying electric field that accelerates electrons, and that acceleration enables the plasma to be established as disclosed in U.S. Pat. No. 4,431,898 issued to Reinberg et al.
In capacitively coupled systems, an RF field may be produced between a pair of opposed electrodes, wherein the electrodes are nominally parallel to the surface of the semiconductor wafer(s) to be processed. In fact, the semiconductor wafer(s) to be processed are often located on one of the electrodes. An example of such a plasma processing system is disclosed in U.S. Pat. No. 4,209,357 (hereinafter “the '357 patent”), issued to Gorin et al., which is herein incorporated by reference.
Plasma processors often require at least one feed gas to be introduced into the plasma processing chamber. Conventionally, feed gases are introduced into the plasma chamber through gas inlet tubes which are located around the periphery of the region in which the plasma is to be established. A distribution manifold may also be used to introduce gas into a plasma processing chamber. Examples of such plasma processors are disclosed in the '357 patent, U.S. Pat. No. 5,624,498 issued to Lee et al.; U.S. Pat. No. 5,614,026 issued to Williams; and U.S. Pat. Nos. 5,614,055 and 5,976,308 both issued to Fairburn et al. The contents of all of the above-referenced patents are incorporated herein by reference.
Feed gas distribution systems of known plasma processors that are fixed and have only low pressure injection with respect to the plasma processing chambers have drawbacks. One such system is shown in FIG. 1 in which continuous gas flow 102 passes through a shower head gas injection system including an array of continuous flow shower head orifices 100 within an upper electrode 124. The gas flow 102 effuses through the shower head orifices into the low-pressure (vacuum) region and interacts with the substrate 114.
A shortcoming of known gas injection systems is the lack of gas directivity, particularly when interacting with the substrate 112. In other words, the gas velocity angular distribution is broad (or the gas velocity directivity is isotropic). In known systems useful for etch processes, a low mass flow rate (i.e. approximately 500 sccm argon equivalent) suitable for a low-pressure process (i.e. 1 to 50 mTorr) is generally preferred. Moreover, known gas injection systems include a shower head including a plurality of (usually 0.5 to 1 mm diameter) injection orifices (typically several hundred orifices). In addition to flow rate and pressure processing conditions, the injection design must be sufficient to affect a uniform gas flow for large substrate areas (e.g., 200 to 300 mm or larger). The combination of the gas injection design and the conditions suitable for etch processes (described above) leads to a low injection total pressure (e.g., of order 1 Torr or less). The injection total pressure refers to the gas pressure within the gas injection cavity upstream of the injection orifices during stagnation conditions. In order to increase the total pressure for gas injection, one must significantly increase the mass flow rate, reduce the number of injection orifices and/or decrease the orifice size; any of which are generally impractical in conventional systems.
Another drawback of known feed gas distribution systems is that the injection systems are not designed with nozzle orifice geometries suitable to constrain the rate of gas expansion into the low pressure vacuum environment.
Yet another drawback of known feed gas distribution systems is the inadequate continuous high mass flow rate (which is necessary to achieve high injection total pressures). This requirement is further exacerbated at lower pressures. At present, state-of-the-art vacuum pumping technology is inadequate to produce a pumping speed at the processing region sufficient to accommodate the necessary high throughputs at low pressure for high pressure injection.